Apparatus and method for measuring concentrations of scale-forming ions

ABSTRACT

This invention relates to methods and apparatus for determination of ion concentrations, particularly in downhole water from hydrocarbon wells, aquifers etc. It is useful in a wide range of applications, including predicting the formation of scale and fingerprinting waters from different sources. More particularly, the invention relates to the use of ligands whose electronic configuration is altered by the binding of the scaling ions within a water sample. These alterations are detected electrochemically by applying varying potential to electrodes and measuring current flow as potential is varied, from which is derived the concentration of scaling ions in the fluid.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/564,939, filed 17 Jul. 2006, entitled “Apparatus and Method forMeasuring Concentrations of Scale-Forming Ions” which is incorporatedherein by reference and which was based on PCT applicationPCT/GB2004/003040, filed 12 Jul. 2004, entitled “Apparatus and Methodfor Measuring Concentrations of Scale-Forming Ions”. This applicationclaims priority under 35 U.S.C. 119 from Great Britain Patent No.2404252, filed 24 Jul. 2003, entitled “Measuring concentrations of ionsin downhole water”.

FIELD OF INVENTION

This invention relates to the determination of ion concentrations indownhole water from hydrocarbon wells, aquifers etc. This is useful in awide range of applications, including predicting the formation of scaleand fingerprinting waters from different sources.

BACKGROUND

The prediction of the location and type of mineral scale that may formaround or within the production or surface facilities of an oil well isan important factor both in the design of the well and the formulationof strategies to cope with the mineral scale.

Current methods for predicting mineral scale formation involve theretrieval of samples from downhole, which are then either analyzed atthe surface or else sent off to laboratories for analysis. Errors anddelays can arise from this ex situ analysis.

Electrochemical methods have previously been developed for themeasurement of the concentration of a number of different metal ions,and some have been deployed in shallow boreholes, lakes and oceanwaters. However, the application of these methods to oilfield operationshas been limited, as the high temperatures (up to 175 Celsius) andpressures (up to 1500 bar) common to most reservoirs, make their useimpractical. Furthermore, many electrochemical methods are not able todistinguish between the principal metal ions (Ca²⁺, Ba²⁺ and Sr²⁺}responsible for scale formation. This problem is compounded by the lowconcentrations of these ions (about 10 s mg/L) in formation water whichis often highly saline.

The ability to rapidly and conveniently distinguish scaling ions mayalso find application, for example, in fingerprinting waters flowinginto a hydrocarbon well from different producing zones. Thisinformation, which is indicative of connectivity between different zonesof a producing well, may allow the optimization of production strategiesfor recovering the oil in place.

SUMMARY OF THE INVENTION

An object of the invention is to provide improved methods for themeasurement of the scaling ions, which are suitable for use in situ i.e.in a continuous connection to a flow of fluid.

Accordingly, a first aspect of the invention provides an apparatus fordetermining the concentration of scaling ions in downhole water; theapparatus comprising a ligand which binds scaling ions from a flowingfluid, which could be downhole water, said ligand having an electronicconfiguration which is altered on binding of a scaling ion, and adetector for determining alterations in said electronic configuration,the amount of said alterations being indicative of the concentration ofthe scaling ion in the sample.

Preferably the ligand is contained within an electrochemical cell andchanges in the electroactivity of the ligand are determined, for exampleamperometrically or voltammetrically. In other embodiments, the bindingof a scaling ion may alter the fluorescent properties of the ligand.Changes in the fluorescence of the ligand upon binding of the ligand maybe determined using any of a range of conventional techniques.

The apparatus may comprise a single ligand which binds specifically to asingle scaling ion, such that changes in the electronic configuration ofthe ligand are directly related to the concentration of the scaling ionin the sample water.

More preferably, the apparatus may contain two or more differentligands, for example three, four, or five or more. Alterations in theelectronic configuration of each ligand may be determined independently,either simultaneously or sequentially.

In some embodiments, each ligand may bind specifically to a differentscaling ion. Changes in the electronic configuration of each ligand aredirectly related to the concentration of the corresponding scaling ionin the sample water.

In other embodiments, each ligand may bind to two or more differentscaling ions. Changes in the properties (i.e. the electronicconfiguration) of each ligand are directly related to the concentrationin the sample water of the two or more scaling ions to which that ligandbinds. The different electronic response of the ligand to different ionscan be translated into a respective concentration measurements, forexample by locating the peaks in a voltagram.

Alternatively, each ligand may bind to a different combination ofscaling ions such that the concentration of each individual scaling ionin the sample water may be calculated from the measurements determinedfor two or more different ligands.

An advantage of the apparatus is that it allows in situ analysis to beperformed, thereby avoiding the problems associated with transportingsamples to the surface for ex situ analysis. The present invention ispartly based on the realisation that electrochemical techniques can beadapted for performance downhole, i.e. in relatively demanding andhostile conditions.

Preferably the detector is operably connected to a processor fordetermining the concentration of scaling ions from the current orpotential in the cell. In some embodiments, the apparatus is adapted foruse downhole (i.e. in a hydrocarbon well or aquifer). The processor mayalso be adapted for use downhole, or alternatively it may be intendedfor remote installation e.g. at the surface. For example, the processormay be a suitably programmed computer.

A further aspect of the invention provides for the use of apparatus asdescribed herein for in situ measurement of scaling ion concentration.

In another aspect the invention provides a method of monitoring theconcentrations of scaling ions in downhole water comprising;

contacting a sample of downhole water with a ligand which selectivelybinds scaling ions, wherein the binding of scaling ions in said sampleto the ligand alters the electronic configuration of the ligand;

measuring changes in the electronic configuration of the ligand; and,

determining the concentration of said scaling ion from said changes inelectronic configuration.

BRIEF DESCRIPTION OF THE FIGURES

Specific embodiments of the invention will now be described withreference to the following drawings, in which:

FIGS. 1A and 1B show examples of an apparatus according to theinvention.

FIGS. 2 to 5 show examples of ligands suitable for use in accordancewith the invention.

FIG. 6 shows a voltagram measured using a ligand of FIG. 5 in anion-free fluid and a fluid with Ba-ions.

FIG. 7 shows a flow diagram of a method in accordance with an example ofthe present invention.

FIG. 8 shows an example of a scale sensor in a downhole application.

DETAILED DESCRIPTION OF THE INVENTION

In general terms, the present invention relates to the measurement ofconcentration of ions in downhole water, in particular ions responsiblefor scale formation by means of changes in the electronic configurationof a ligand which binds scaling ions. A preferred approach involves theuse of an electrochemical cell containing a ligand whose electroactivitychanges on binding a scaling ion. Changes in ligand electroactivity uponion binding alter the electrochemical properties of the cell and may bemeasured using a detector. Other approaches may comprise the use of aligand whose fluorescent properties change on binding of a scaling ion.

Downhole water may be comprised within a production fluid from ahydrocarbon well or reservoir, which may comprise hydrocarbons, drillingmud etc. The downhole water may, for example, be connate water.

Scaling ions are ions which are responsible for the formation of scale.The principal scaling ions in downhole water are Ca²⁺, Ba²⁺ and Sr²⁺. Asuitable ligand may bind selectively to one or more of these scalingions e.g. a ligand may bind to Ca²⁺, Ba²⁺ and Sr²⁺. Preferably, a ligandshows substantially no binding to other ions.

In some embodiments, the ligand may have a different binding affinityfor each of the three principal scaling ions (Ca²⁺, Ba²⁺ and Sr²⁺),allowing the levels of each individual ion in the downhole water to bedetermined. Discrimination between different ligands may be achieved,for example, by determining the characteristic redox properties of eachligand at different potentials.

The ligand may be present in the cell in an aqueous solution at aconcentration of 0.1 to 10 mM, preferably 1 to 10 mM, or may bedispersed within a porous polymer membrane.

Ligands suitable for use in accordance with the invention are stable andable to bind scaling ions under downhole conditions, for example at hightemperature (e.g. up to 175° C.) and pressure (e.g. up to 1500 bar).

One class of suitable ligands have the formula (I):

where R1 is a C₁₋₅ alkyl (including, e.g. unsubstituted C₁₋₅alkyl andsubstituted C₁₋₅ alkyl) or C₁₋₈ aryl (including, e.g. unsubstituted C₁₋₈aryl and substituted C₁₋₈ aryl); and,

R2 to R9 may independently be H, halogen (F, Cl, Br, I); C₁₋₅ alkylgroup; O—C₁₋₅ alkyl group; COOH; NH₂; —CONH₂; CO—C₁₋₅ alkyl group; or afluorophore group such as carboxy-X-rhodamine (ROX),tetramethylrhodamine (TAMRA) and fluorescein (FAM).

“C₁₋₅ alkyl” pertains to a monovalent moiety obtained by removing ahydrogen atom from a C₁₋₅ hydrocarbon compound having from 1 to 5 carbonatoms, which may be aliphatic or alicyclic, or a combination thereof,and which may be saturated, partially unsaturated, or fully unsaturated.

Examples of suitable ligands according to formula I are shown in FIG. 2.

In some embodiments, the aromatic rings of suitable ligands may comprisesubstitutions in the ortho, meta or para positions (i.e. at one or moreof positions R2 to R9), in order to shift the redox features of a ligandto allow scanning for the different ions in well-separated spectralwindows, in order to prevent interference.

For the purpose of this invention the above class of ligands arereferred to asO,O′-Bis(2-aminophenyl)ethyleneglycol-N,N,N′,N′-tetraacetic acid orBAPTA derivatives.

Other suitable ligands may include cryptands (Lehn & Sauvage (1975) J.Am. Chem. Soc. 97 23 6700), for example a ligand shown in FIG. 3, andthymolphtalein and their derivatives (Qing and Yuying (1987) Talanta 346 555), for example ligands shown in FIG. 4. Other suitable ligands mayinclude neutral ionophores (Simon et al Anal. Chem. 1985, 57, 2756),specific crown ethers (D. J. Cram et al J. Am. Chem. Soc., 1973, 95,3021) or antibiotics such as valinomycin.

A further ligand of the cryptand family is shown in FIG. 5. The cryptandis derivatized by a redox-active group or moiety M. The entity M can beselected for example from a group consisting of Fe, Ru, Co, V, Cr, Mo,and W and n and m can range from 1 to 3.

For the purpose of this invention, ligands of the type of FIGS. 3 and 5are referred to as crypt and derivatives.

The apparatus may further comprise a porous membrane or porous electrodeblock which allows ions within the downhole water to pass into the cellto contact the ligand. A suitable porous membrane may be made of zeoliteor a ceramic material. A block may be made of epoxy material as basematerial.

The membrane may be contacted with discrete samples or batches ofdownhole water or the membrane may be contacted with a continuous flowof downhole water.

The apparatus may comprise one or more liquid guidance channels todirect downhole water to the membrane and to remove downhole water aftercontact with the membrane.

The detector may comprise one or more electrodes which contact theligand. Various arrangements of electrodes may be used as isconventional in electrochemistry.

Conveniently a three-electrode arrangement consisting of a workingelectrode, a reference electrode and a counter electrode may be used.Preferably, the working electrode is composed of a material resistant tofouling, such as boron-doped diamond or glassy carbon, the counterelectrode is platinum and the reference electrode is Ag/AgCl. Othersuitable electrode materials, such as AgI, are known to those skilled inthe art.

The electrodes may be used to detect changes in the electroactivity ofthe one or more ligands. For example, electroactivity changes caused bythe presence of scaling ions may alter the current flow or voltagebetween electrodes. Current or voltage may be detected or measured bythe detector. For example, the potential of the electrodes may be variedand the current measured or vice versa. The current or potentialdifference associated with the electroactivity of each of the one ormore ligands may be measured by the detector and correlated with theconcentration of scaling ions in the downhole water sample. In thepresence of the target ions, the peak current(s) should increase,proportional to the concentration of the target species. A power sourcemay be connected to the electrodes to drive the current between theelectrodes. The power source may be an integral part of the apparatus,and, for example, may be comprised within the detector. In otherembodiments, the power source may be separate from the apparatus andconnectable thereto. The apparatus may comprise appropriate circuitryfor connection to the power source.

The ligand may be contained within the apparatus in any of a number ofways. In some embodiments, the ligand may be dispersed in an aqueoussolution within a chamber of the apparatus. In other embodiments, theligand may be dispersed within a porous polymer membrane. Binding of thescaling ions by the ligand occurs within the pores of the membrane andresultant changes in current or potential are detected by circuitryconnected directly to the membrane via the working, counter andreference electrodes. The use of a porous membrane is convenient inallowing the miniaturisation of the voltammetric or amperometric sensor,thus leading to faster response times, lower consumption of reagents andlower unit costs.

In other embodiments, the ligands may be attached to conducting solidparticles, such as carbon or a metal (e.g., gold), which areincorporated into the surface of one or more of the electrodes,preferably, the working electrode. The accumulation of particles withattached ligand forms a conducting porous electrode with ligand attachedto the walls of the pores. Suitable techniques for fixing the particlesto the electrode surface include epoxy resin adhesion or abrasiveimmobilisation. A porous electrode for hydrogen sulfide determination,for example, is described in co-pending published United Kingdomapplication GB-A-2391314.

For example, the ligand (I) above may be designed such that group R8 isan amine (—NH₂), which can be reacted with nitrous acid to form thediazonium ion —N⁺≡N and subsequently coupled to carbon particles byreduction of the diazonium group by hypophosphorous acid. The ligand isthus chemically bonded to the carbon particles and these can beincorporated into the working electrode 4 as described above. In otherembodiments, the ligand (I) above may be coupled to gold particles withone of the groups R2 to R9 being either an amine (—NH₂) or a thiol(—SH).

As described above, the detector may be operably connected to aprocessor that determines the concentration of scaling ions in thesample from the current or potential difference measured by thedetector. The processor may be separate from or part of the detector.The processor may also be adapted for use under downhole conditions(i.e. high temperature, high pressure and high salinity). Alternatively,it may be intended for remote installation e.g. at the surface. Forexample, the processor may be a suitably programmed computer.

The measurement of scaling ion concentrations as described herein may beuseful in downhole sampling, production logging to characterize flowinto the well, and thereby aid remediation or production strategies, andin permanent monitoring applications, where the build up of scale orwater breakthrough/flooding of the reservoir might be gauged.

FIG. 1A shows a cross-sectional diagram of an apparatus according to oneembodiment of the invention. The apparatus is shown separated into anupper and a lower part as in a stage of being assembled. Inlets 11 andoutlets 12 for sampling downhole water are indicated by arrows pointingin the direction of the flow. The sample water contacts a membrane 13Awhich allows the passage of ions into the cell 14. The ligand solutionin the cell 14 is contacted by a Ag/AgCl reference electrode 15, aplatinum ring counter electrode 16 and a glassy carbon working electrode17. The electrodes 15, 16 and 17 detect changes in the electroactivityof ligand in the cell 14 which are related to scaling ion concentration.

In the variant of FIG. 1B, a scale sensor 20 is shown coupled to aflowline 23. The body 21 of the sensor is fixed into the end section ofan opening 22. The body carries a microporous epoxy matrix 211 embeddingthe catalysts 214 and contacts 212 that provide connection points tovoltage supply and measurement through a small channel 221 at the bottomof the opening 22. A sealing ring 213 protects the contact points andelectronics from the wellbore fluid that passes under operationconditions through the sample channel 23.

In an example according to an embodiment of the invention, the fourligands (2A-2D) shown in FIG. 2 may be present in solution in cell 14 orembedded in block 211.

These ligands have different binding properties; ligand 2A binds Ca²⁺,Sr²⁺ and Ba²⁺; ligand 2B binds Ca²⁺ and Sr²⁺, ligand 2C binds Sr²⁺ andBa²⁺ and ligand 2D binds Ba²⁺.

The level of Sr²⁺ in the sample water may be determined, for example, bymeasuring the alterations of the electroactivities of ligands 2C and 2Din the cell and then subtracting the value obtained for ligand 2D fromvalue obtained for ligand 2C, to provide a value which represents theconcentration of Sr²⁺. (i.e. 2C−2D=[Sr²⁺]). As above the figure label istaken as a representative of the respective ligand and/or theconcentration measurement associated with it.

The level of Ca²⁺ in the sample water may be determined by measuring thealterations of the electroactivities of ligands 2B, 2C and 2D in thecell. The values for ligands 2B and 2D are added together and the valueobtained for ligand 2C is subtracted from this combined figure, toprovide a value which represents the concentration of Ca²⁺. (i.e.2B+2D−2C=[Ca⁺⁺]).

The level of Ba²⁺ in the sample water is determined by measuring thealterations of the electroactivities of ligands 2A and 2B in the celland then subtracting the value obtained for ligand 2B from the valueobtained for ligand 2A, to provide a value which represents theconcentration of Ba²⁺. (i.e. 2A−2B=[Ba²⁺] or ligand 2D).

The chemical structure of further examples of ligands are shown in FIGS.3 and 4. The indices n and m of the ligand in FIG. 3 can be 1 or 2. FIG.5 shows the example of a crypt and modified with a redox active moiety.The entity M can be selected from a group consisting of Fe, Ru, Co, V,Cr, Mo, and W and n and m can range from 1 to 3.

In FIG. 6 there is shown the response of the ligand of FIG. 5 with M=Feand n=m=2 to the presence of Ba²⁺. The solid line 51 is the typicalelectrochemical response of the pure ligand, whereas the dashed line 52is the same response in the presence of Ba cations. The ligand of FIG. 5is sensitive to more than one species of scale-forming ions and thepresence of different ions can be readily detected from determining thepeak locations in the voltagramm. This ligand thus alleviates the needto use multiple ligands. Square wave voltametry may be used instead ofthe shown full cycle voltametry.

The flowchart of FIG. 7 summarizes steps of a method exemplary of thepresent invention, including the step 71 of contacting a sample of forexample downhole water with a ligand which selectively binds scalingions, the step 72 of measuring changes in the electronic configurationof the ligand, and the step 73 of determining the concentration of thescaling ions from the change in electronic configuration. The results ofthe measurement may be fed into a model 74 that predicts the built-up ofscaling in tubulars and other flow exposed equipment, for exampleproduction tubing or downhole pumps.

An application of the sensor is illustrated in FIG. 8. It shows aVenturi-type flowmeter 810, as well known in the industry and describedfor example in the U.S. Pat. No. 5,736,650. Mounted on production tubingor casing 812, the flowmeter is installed at a location within the well811 with a wired connection 813 to the surface following knownprocedures as disclosed for example in the U.S. Pat. No. 5,829,520.

The flowmeter consists essentially of a constriction or throat 814 andtwo pressure taps 818, 819 located conventionally at the entrance andthe position of maximum constriction, respectively. Usually the Venturiflowmeter is combined with a densiometer 815 located further up- ordownstream.

The novel scale sensor 816 is preferably located downstream from theVenturi to take advantage of the mixing effect the Venturi has on theflow. A recess 817 protected by a metal mesh provides an inlet to theunit.

During production wellbore fluid enters the recess 817 and issubsequently analyzed using sensor unit 816. The results are transmittedfrom the data acquisition unit to the surface via wires 813.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

1. An apparatus for determining the concentration of scale-forming ions;the apparatus comprising: a ligand which binds scaling ions in a fluid,said ligand having an electronic configuration which is altered onbinding of a scaling ion, and a detector comprising a plurality ofelectrodes and means for applying varying potential to the electrodesand measuring current flow, the apparatus being configured to place theligand in the vicinity of a flow of said fluid and in the vicinity ofsaid electrodes such that alterations in the electronic configurationthrough binding of a scaling ion, change the flow of current in responseto applied potential, the amount of said alterations being indicative ofthe concentration of the scaling ion in the sample.
 2. An apparatusaccording to claim 1 wherein the scaling ion is selected from the groupconsisting of Ca²⁺, Ba²⁺ and Sr²⁺ ions.
 3. An apparatus according toclaim 1 wherein the ligand is immobilised on conducting particlesattached to one or more of said electrodes.
 4. An apparatus according toclaim 3 wherein said conducting particles are carbon or metal particles.5. An apparatus according to claim 4 wherein the metal particles aregold particles.
 6. An apparatus according to claim 3 wherein saidconducting particles with immobilised ligands thereon form a conductingporous electrode.
 7. An apparatus according to claim 1 wherein theligand comprises oxygen and/or nitrogen.
 8. An apparatus according toclaim 1 wherein the ligand binds two or more different scaling ions andgenerates a different electronic configuration and different changes tothe flow of current in response to binding of different scaling ions. 9.An apparatus according to claim 1 comprising two or more differentligands, said detector being adapted to determine alterations in theelectronic configuration of each of the different ligands independently.10. An apparatus according to claim 9 wherein each of the said two ormore ligands binds to a different combination of scaling ions.
 11. Anapparatus according to claim 1 further comprising a processor forcalculating the concentration of one or more scaling ions in the fluidfrom measurements of current flow in response to applied potential. 12.An apparatus according to claim 1 further comprising a porous membranewhich allows ions from the fluid to contact the ligand.
 13. An apparatusaccording to claim 12 wherein the membrane is ceramic or zeolite.
 14. Anapparatus according to claim 1 wherein the ligand is embedded in a blockof porous material for exposure to a fluid flow.
 15. An apparatusaccording to claim 1 wherein the ligand comprises a redox-active group.16. An apparatus according to claim 15 wherein the ligand comprises aportion binding a scaling ion and a redox-active group attached thereto.17. An apparatus according to claim 1 wherein the fluid is a wellboreeffluent.
 18. An apparatus according to claim 1 wherein the fluid stemsfrom a production flow from a wellbore.
 19. An apparatus according toclaim 1 adapted to be placed in a subterranean location.
 20. A method ofmonitoring the concentrations of scaling ions comprising; contacting afluid flow with a ligand which selectively binds scaling ions, whereinthe binding of scaling ions to the ligand alters the electronicconfiguration of the ligand; applying varying potential to electrodesplaced in the vicinity of the ligand such that alterations in theelectronic configuration through binding of a scaling ion change theflow of current in response to applied potential, measuring the currentflowing in response to applied potential, and thereby measuringalterations in the electronic configuration of the ligand; and,determining the concentration of said scaling ion from the measurementsof current flow.
 21. A method according to claim 20 wherein the scalingions are selected from the group consisting of Ca²⁺, Ba²⁺ and Sr²⁺ ions.22. A method according to claim 20 wherein the ligand comprises aredox-active group.
 23. A method according to claim 22 wherein theligand comprises a portion binding a scaling ion and a redox-activegroup attached thereto.
 24. A method according to claim 20 wherein theligand binds two or more different scaling ions and generates adifferent electronic configuration and different changes to the flow ofcurrent in response to binding of different scaling ions.
 25. A methodaccording to claim 20 comprising contacting the fluid with two or moredifferent ligands and determining alterations in the electronicconfiguration of each of the two or more different ligands.
 26. A methodaccording to claim 25 wherein each of the said two or more ligands bindsto a different combination of scaling ions.
 27. A method according toclaim 20 including the step of monitoring the production of a wellbore.28. A method according to claim 20 including the step of predicting thescaling of hydrocarbon production tubulars or equipment.
 29. A methodaccording to claim 20 including the step of monitoring the scaling ofhydrocarbon production tubulars or equipment in a downhole location.